Coinheritance of HNF1A and glucokinase variants in maturity-onset diabetes of the young

Summary Maturity-onset diabetes of the young (MODY) is a group of monogenic forms of diabetes mellitus characterized by early-onset diabetes with dominant inheritance of beta-cell dysfunction. There are few reports of the coinheritance of glucokinase (GCK) and hepatocyte nuclear factor 1 alpha gene (HNF1A) variants underlying MODY in patients. Herein, we describe a case involving combinations of monoallelic GCK and HNF1A variants associated with MODY. A 10-year-old Japanese girl with a three-generation family history of diabetes without obesity showed high levels of urinary glucose during a school screening test. Her glucose metabolism profile revealed 124 mg/dL of fasting glucose, 6.9% glycated hemoglobin (HbA1c), and 2.78 ng/mL of C-peptide immunoreactivity levels. In a 75-g oral glucose tolerance test, her base glucose, peak glucose, insulin resistance, and homeostasis model assessment of beta cell function levels were 124 mg/dL, 210 mg/dL (120 min), 1.71, and 33%, respectively. Based on the clinical phenotype of GCK-MODY, alimentary and exercise therapy without oral hypoglycemic agents were used to maintain her fasting glucose and HbA1c levels. We explored the coinheritance of MODY with GCK and HNF1A variants in this and past cases and found that careful clinical follow-up is required to firmly establish phenotypic features. Moreover, the accumulation of data on genetically confirmed MODY associated with the coinheritance of GCK and HNF1A variants will be useful for understanding genotype–phenotype correlations. Learning points MODY is a group of monogenic forms of diabetes mellitus characterized by early-onset diabetes with the dominant inheritance of beta-cell dysfunction. MODY2 and MODY3 caused by heterozygous loss-of-function variants in the glucokinase (GCK) and hepatocyte nuclear factor 1 alpha (HNF1A) genes, respectively, are the most common forms of the disease. Few cases of MODY have previously been reported as being associated with the coinheritance of GCK and HNF1A variants. Careful clinical follow-up is required to firmly establish phenotypic features in the coinheritance of MODY with GCK and HNF1A variants. The accumulation of data on genetically confirmed MODY associated with the coinheritance of GCK and HNF1A variants will be useful for understanding genotype–phenotype correlations.


Learning points
• MODY is a group of monogenic forms of diabetes mellitus characterized by early-onset diabetes with the dominant inheritance of beta-cell dysfunction.• MODY2 and MODY3 caused by heterozygous loss-of-function variants in the glucokinase (GCK) and hepatocyte nuclear factor 1 alpha (HNF1A) genes, respectively, are the most common forms of the disease.• Few cases of MODY have previously been reported as being associated with the coinheritance of GCK and HNF1A variants.• Careful clinical follow-up is required to firmly establish phenotypic features in the coinheritance of MODY with GCK and HNF1A variants.• The accumulation of data on genetically confirmed MODY associated with the coinheritance of GCK and HNF1A variants will be useful for understanding genotype-phenotype correlations.

Background
Maturity-onset diabetes of the young (MODY) is a group of monogenic forms of diabetes mellitus characterized by early-onset diabetes with dominant inheritance of beta-cell dysfunction.Additional features of MODY include negative pancreatic autoantibodies and persistently detectable C-peptide levels, which differentiate it from type 1 diabetes and non-obesity and type 2 diabetes.To date, 14 different causative genes have been proposed for MODY.MODY2 and MODY3 caused by heterozygous loss-of-function variants in the glucokinase (GCK) and hepatocyte nuclear factor 1 alpha (HNF1A) genes, respectively, are the most common forms of the disease.Clinical manifestations of MODY2 include a moderate increase in fasting glycemia, typically from birth.Diabetes-associated complications are unlikely in patients with MODY2, and they rarely require pharmacological treatment.MODY3 is usually diagnosed in adolescence or early adulthood, requires pharmacological treatment, and often leads to late-onset microvascular complications.Hundreds of variants have been identified in GCK and HNF1A (1).
Typically, only one gene is thought to be causative of MODY, and a few cases of MODY have been reported to be associated with the coinheritance of GCK and HNF1A variants (1,2,3,4,5).Data on the clinical follow-up of individuals with the coinheritance of GCK and HNF1A in MODY are scarce.In addition, the coinheritance of GCK and HNF1A variants was found to be extremely rare in a recent study of cancer multi-gene panel testing (6,7).Herein, we report a case of MODY with coinheritance of monoallelic variants in GCK and HNF1A.

Case presentation
The patient was a 10-year-old Japanese girl born with a weight of 3110 g at 40 weeks of gestation.High levels of urinary glucose were first detected during a school screening examination at the age of 10.She had a threegeneration family history of diabetes (Fig. 1A).On her paternal side, her father, grandfather, and aunt had been diagnosed with diabetes but did not take oral hypoglycemic drugs.Her father was diagnosed with diabetes at the age of 23 years.On her maternal side, her grandfather had diabetes, but her mother and grandmother have not been diagnosed with diabetes.She was non-obese (height: 141.4 cm; weight: 38.8 kg), and no acanthosis nigricans was observed at the back of her neck.

Treatment
She received exercise and alimentary therapy, with no oral administration of hypoglycemic agents.

Outcome and follow-up
The GCK p.Arg191Trp is classified as pathogenic (VCV000426122.21) in the ClinVar database (https:// www.ncbi.nlm.nih.gov/clinvar/) with a minor allele frequency of < 0.01.HNF1A c.1624-15G>A is classified as having conflicting interpretations of pathogenicity (VCV000036806.6) in the ClinVar database.The minor allele frequencies of HNF1A c.1624-15G>A are 0.0002 and 0.0053 in the 1000 Genomes Project database (https://www.internationalgenome.org/1000-genomesbrowsers) and the 4.7K Japan allele and genotype frequency panel (https://jmorp.megabank.tohoku.ac.jp/), respectively.In silico analysis predicted HNF1A p.Ala544Thr, a minor transcript with an amino acid change, to be benign (0.001) by Polyphen-2 (http:// genetics.bwh.harvard.edu/pph2/index.shtml),tolerated Family pedigree and sequence analysis of GCK and HNF1A in the patient and parents.A) In the family pedigree, her father was diagnosed with diabetes at the age of 23 years.On the paternal side, the patient's father, grandfather, and aunt were diagnosed with diabetes.On the maternal side, the patient's grandfather had diabetes, but her mother and grandmother have not been diagnosed with diabetes.All family members have not needed oral hypoglycemic drugs and have been treated with diabetes dietary and exercise therapy, resulting in no diabetic complications.B) The patient harbored compound heterozygous variants GCK c.

Discussion
This report describes a case of MODY with a confirmed coinheritance of monoallelic variants of GCK and HNF1A.Our observation points to the notion that careful clinical follow-up is required to firmly establish phenotypic features in the coinheritance of MODY with GCK and HNF1A variants.
A comparison of clinical features previously reported patient and currently reported by one of the coinheritance of MODY with GCK and HNF1A variants is tabulated.We identified nine cases reported in five studies (1, 2, 3, 4, 5) (Table 1).There were no significant sex differences among the patients (males, 5; females, 3; unknown, 1).Several of these patients were diagnosed at a young age with mildly elevated glucose and HbA1c levels and persistently detectable C-peptide levels ( , closely resembling MODY2 in that the patient's diabetes was well controlled with exercise and dietary therapy, similar to our patient.In contrast, two adult patients had received oral administration of drugs (1, 2), resulting in a metabolic phenotype that was not entirely consistent with the GCK variant.Furthermore, although OGTT results were not available in the previously reported patient, our patient had high glucose levels at > 120 min, which is not a typical MODY2 phenotype; a characteristic feature of MODY3 in OGTT results is usually peak glucose levels at > 120 min (8).
Careful clinical follow-up is needed to determine the phenotype, especially in pediatric patients, of the coinheritance of MODY involving GCK and HNF1A variants, which may reflect having some combined effect on glucose metabolism.
Variant interpretation is important for predicting clinical phenotypes and elucidating disease mechanisms.The GCK p.Arg191Trp allele appears to be a relatively common variant site in MODY2, identified in previous studies reporting GCK MODY (9).Other variants located in the 191 position of GCK have been suggested to be pathogenic (10), while GCK p.Arg191Trp is pathogenic in ClinVar so appears to be causative of diabetes in the patient's father's familial history.Consequently, GCK p.Arg191Trp is classified as 'Pathogenic' (PS1, PM1, PM5, PP1, PP3, PP4) in the American College of Medical Genetics (ACMG) guidelines.It remains unclear whether the identified HNF1A c.1624-15G>A variant influences the onset of MODY.This variant was previously reported in a patient with MODY (11) without supporting evidence of impact.In several in silico analyses, the pathogenicity of this variant differed across models.Consequently, the HNF1A c.1624-15G>A variant was classified as 'likely benign' (PP5, BS1, BP2) in the ACMG guidelines.However, the clinical phenotype of HNF1A variants is known to be highly variable, even within the same family (12), and has a variety of effects ranging from weak to highly penetrant effects in previous functional studies (13).The genome-wide association studies have identified genetic susceptibility variants for diabetes (14).Therefore, HNF1A may represent a phenotype that cannot be explained by the pathogenicity assessment of simple variants alone.
The molecular endocrinological mechanisms of MODY associated with coinheritance are complex and may be explained by the following mechanisms: (i) GCK and HNF1A exist in the same pathway in beta-cell function (15).HNF1A progressively impairs betacell functionality, eventually reducing the response to physiological stimuli.This impairment results in a dominant effect on GCK (1).Low-impact variants are known to be involved in the same pathway development of metabolic syndromes reflecting the small effect of the coinheritance of variants, including diabetes, because of the accumulation of burden ( 16); (ii) The transcripts of HNF1A include the Matched Annotation from the NCBI and EMBL-EBI (MANE) Select transcript 'NM_000545.5' and minor transcript 'NM_001306179.2'.MANE Select transcripts are biologically supported reference sequences that serve as optimal transcripts for clinical variation analysis, capturing nearly all pathogenic variants.However, some alternative protein isoforms are also biologically relevant (17,18).The complexity of the genetic and molecular landscape of MODY could be dependent on the different transcripts.Further studies are needed to determine the relationship between genetic variants and molecular mechanisms.
Our study has several limitations.First, we focused on known monogenic diabetes genes using an NGS panel, so it was not possible to rule out the influence of variants outside the target of NGS panel analysis.Secondly, we did not conduct a functional analysis of the detected variants.In vitro, functional analysis and detailed family studies are needed to clarify the role of this variant.In addition, the HNF1A variants were classified as 'likely benign' by the ACMG guidelines, which may have identified functional SNPs with limited effects as potentially causative.However, NGS panel analysis can at least resolve previously reported pathogenic variants associated with the diabetes phenotype and can be considered a trigger for careful follow-up clinical investigation.
In conclusion, our case may provide useful information to elucidate the genotype-phenotype correlations of MODY.The future identification of genotypes could be helpful in predicting long-term prognosis in MODY patients.

Table 1 Current
and previous GCK and HNF1A variants in maturity-onset diabetes of the young.